In association with a recent noticeable development in a portable electronic equipment, communication equipment and the like, development of secondary batteries with high energy density is strongly requested from the viewpoint of the economic efficiency and reduction in size and weight of the equipment. Currently available secondary batteries with high energy density include a nickel-cadmium battery, a nickel hydrogen-battery, a lithium ion secondary battery, a polymer battery and the like. Among these batteries, demand for the lithium ion secondary battery is strongly growing in the power source market due to its dramatically enhanced life and capacity, compared with the nickel cadmium battery or nickel-hydrogen battery.
FIG. 1 is a view showing a configuration example of a coin-shaped lithium ion secondary battery. The lithium ion secondary battery includes, as shown in FIG. 1, a positive electrode 1, a negative electrode 2, a separator 3 impregnated with electrolyte, and a gasket 4 which seals what is contained in the battery while maintaining the electric insulation between the positive electrode 1 and the negative electrode 2. When charging and discharging are performed, lithium ions reciprocate between the positive electrode 1 and the negative electrode 2 through the electrolyte of the separator 3.
The positive electrode 1 includes a counter electrode case 1a, a counter electrode current collector 1b and a counter electrode 1c, and lithium cobaltate (LiCoO2) or manganese spinel (LiMn2O4) is mainly used for the counter electrode 1c. The negative electrode 2 includes a working electrode case 2a, a working electrode current collector 2b and a working electrode 2c, and a negative electrode material used for the working electrode 2c generally includes an active material capable of occluding and releasing lithium ions (negative electrode active material), a conductive auxiliary agent, and a binder.
As the negative electrode active material for lithium ion secondary batteries, conventionally, a composite oxide of lithium and boron, a composite oxide of lithium and transition metal (V, Fe, Cr, Mo, Ni, etc.), a compound including N and O as well as Si, Ge or Sn, Si particle the surface of which is coated with a carbon layer by chemical vapor deposition, and the like are proposed.
However, each of these negative electrode active materials makes deterioration of an electrode significant since dendrite or a passivated compound is generated on the electrode according to repeated charging and discharging, or enhances the expansion or contraction thereof during occlusion and release of lithium ions, although it can improve the charging and discharging capacities to enhance the energy density. Therefore, lithium ion secondary batteries using these negative electrode active materials are insufficient in the maintainability of discharge capacity by repeated charging and discharging (hereinafter referred to as “cycle characteristics”).
Meanwhile, it is conventionally attempted to use powders of generic silicon oxide represented by SiOx (0<x≦2), such as SiO, as the negative electrode active material. The silicon oxide can be a negative electrode active material with a larger effective charging and discharging capacity, since it is lower (less noble) in electrode potential than lithium and can reversibly occlude and release lithium ions without deterioration during charging and discharging such as collapse of a crystal structure by the occlusion and release of lithium ions or generation of an irreversible substance. Therefore, the silicon oxide can be used as the negative electrode active material to obtain a lithium ion secondary battery which is higher in capacity than in a case using carbon and is much better in cycle characteristics than in a case using a high-capacity negative electrode material such as Si or Sn alloy.
When the silicon oxide powder is used as the negative electrode active material, in general, carbon powder or the like is mixed thereto as a conductive auxiliary agent for compensating the low electric conductance of the silicon oxide. This allows the electric conductivity to be secured around a contact portion between the silicon oxide powder and the conductive auxiliary agent. However, at locations far away from the contact portion, the silicon oxide powder is less likely to function as the negative electrode active material since the electric conductivity cannot be secured.
In order to solve this problem, proposed in Patent Literature 1 is a conductive silicon composite for a negative electrode material for use in a non-aqueous electrolyte secondary battery which contains a film of carbon formed on the surface of a particle having a structure in which microcrystals of silicon are dispersed in silicon dioxide (conductive silicon composite), and a method for producing the same.